The present invention relates to the obtainment, by means of a continuous process and in high yields, of hydrates of hydraulic cement binders and silicic, sulfated or aluminous binders. In particular, the present invention describes a new process for the preparation of simple or complex hydrates, in the form of a fine, micron-sized powder which can constitute basic, insert mineral charges, wherein the process is started from simple or complex compounds such as anhydrous substances, in particular, in industries producing cement binders and/or binders for refractory and/or plastering materials.
It is known, that the cements or silicic or aluminous hydraulic binders which are destined to be hydrated in compact forms and are utilized for their characteristic as binders and for the high resistance these binders possess. These transformations realized by all the manufacturers and users of different mixtures of concrete, has led to products wanted for their characteristics, such as compactness, hardness, resistance to compression, binding character. The different studies carried out in this respect, show that the cement grains first become hydrated rapidly after a latency time, called setting time, and then become hydrated more and more slowly. The hydration realized in this manner leads to gels or to finely interlaced crystals which solidify the medium by reducing the possible ionic movements, which has the effect of usually restraining the chemical reactions: the hydration which thus has started quickly, takes a very long time to reach completion, and the resulting aluminous cements, chiefly consisting of hemi- and mono-calcium aluminates, still undergo hydration reaction in their center after about 6 months or one year. As far as the Portland cements are concerned, their hydration reactions last over several tenths of years.
For study or investigation purposes, it is also known to make hydrates of finely divided hydraulic binders in the laboratory. In order to accomplish this, it suffices to carry out the customary hydrations in the presence of an excess of water, with or without agitation and grinding. For an example of the preparations of laboratory samples, reference is had to the "Journal of Chemistry Society" 1950, pages 3,682 to 3,690 (H. F. W. Taylor) and "The Chemistry of Cement and Concrete", 3rd Edition, F. M. LEA, p. 180.
In accordance with all these known processes, one disperses particles of anhydrous material in agitated or still water. This material dissolves slowly and, through chemical reactions, the hydrates precipitate slowly. Also in the cited work of F. M. LEA, pages 180 and 181, it is reported that a complete hydration of the tri-calcium silicate can be obtained in a ball mill, in one or two days and in the presence of an excess of water. The di-calcium silicate, an important constituent of the Portland Cement, has been reported as becoming hydrated in 46 days under these conditions.
The processes mentioned above can be utilized, as a matter of fact, only in the laboratory; starting from a certain quantity of material and after appropriate operations, more or less accelerated by known physical means such as temperature or chemical means such as accelerators. These cited processes allow one to obtain, always in discontinuous manner, the hydrates formed from anhydrous substances. The applicants have therefore carried out numerous tests and investigations, which have made it possible for them to observe that, because the hydrates are the synthesis of several ions stemming from the anhydrous substances and from the water derived from the reacting medium, the hydrates formed during the hydration of the hydraulic binders, formed preferentially at the surface of the least soluble anhydrous particles, a location in the vicinity of which the solubility-product of the compound to be formed is attained rapidly. This indicates that the surfaces of the least reactive anhydrous particles are less and less in contact with the water and that, due to this fact, the kinetics of the reaction of hydration is diminished itself and that the reaction is terminated. The applicants have furthermore observed that only the small cement particles become enveloped by a hydrate layer, which results therein, that their subsequent dissolution is governed by the ionic diffusion across this hydrate layer. This dissolution is thus governed by the well known laws of Fick and, in the course of time, becomes indefinitely slow.
The materials utilized presently as mineral charges are generally the kaolins, the pyrogenous kaolins, the natural or synthetic calcium silicates such as the Wollastonite, the natural or synthetic calcium carbonates, talc, silica, dolomite, barium sulfate. These products are obtained directly in micron size form through washing and granulometric selection, such as in the case of kaolin. Other products are obtained by precipitation in solution, by starting from ions obtained through dissolution of very highly soluble compounds, such as the sodium silicates and aluminates, lime, and aluminum sulfate. In addition, other products, and this is the most common case, are obtained through grinding of natural rocks, by dry or wet ways, up to the point at which the desired degrees of fineness are obtained. In this manner, one obtains the powders in micron size of carbonates, talcs, dolomites, silicas destined to be utilized as charges or in the charges. If one desires to obtain high degrees of fineness in the order of a micron, these grinding operations become very burdensome and very delicate.
Starting from the previously cited teachings and observations, the applicants had the idea to use, for manufacturing of mineral charges of good quality, the action of water on the slightly soluble anhydrous calcium compounds produced in the industries for hydraulic binders and/or binders for refractory and/or plastering materials, with these binders being ground down to medium degrees of fineness not of micron size, but to procure the micron-size fineness required for the basic hydrate charges. This is done in a manner so that the water brings about an attack of the surface of the anhydrous particles through dissolution and the kinetics of the hydration is maintained at a very high level from the first moments of the hydration reactions by subjecting at least the largest particles in the course of hydration agitation, preferably of a violent nature, and preferably in the presence of inert materials. This is done in a manner so as to achieve friction wear to remove hydrates from the surface of the particles and thus make possible the continuous attack by the water of the anhydrous particles. This operation can be done without the involvement of a grinding process, and is, therefore, economical.
The process according to the invention features the steps of hydrating between about 10.degree. and 100.degree. C at least one of the synthesized calcium aluminates prepared specially or stemming from the manufacture of aluminous hydraulic binders, or of refractory cements or of Portland cements, with the particles being ground down to medium degrees of fineness, subjecting at least the large particles to a vigorous agitation during the hydration, drying the formed suspension, and collecting the obtained product.
If needed, lime is added to complete the stoichiometry necessary for the reactions. This addition of lime is generally unnecessary if one treates Portland cements or their clinkers.
As raw materials, the following can be utilized:
synthesized calcium aluminates, prepared for the purpose of the invention or stemming from the manufacture of refractory hydraulic binders, obtained by themselves or in mixtures, namely:
3 CaO Al.sub.2 O.sub.3 ; 12 CaO 7 Al.sub.2 O.sub.3 ; CaO Al.sub.2 O.sub.3 ;CaO 2Al.sub.2 O.sub.3.
If the calcium aluminate is too poor in lime content in relation to the stoichiometric quantity of lime necessary for the reactions which will be specified later on, then lime should be added in anhydrous or hydrated form, in a pure state or in an impure state, with the impurities usually consisting of calcium and magnesium carbonates. As a source of lime, CaO, calcium silicates can be used. In particular, Portland cement is known to liberate lime during the hydration of the calcium silicates which it contains. Thus, mixtures are obtained.
The invention can also apply to the manufacture of hydrates stemming from Portland cements, or from their ground-up clinkers, or from aluminous cements or their ground-up clinkers, consisting principally of calcium aluminates.
Preferably, the starting materials should be white. Products such as white Portland cement and white aluminous cements such as the refractory aluminous cements are available and suitable. All the white products of these types can be utilized, but, as far as the invention is concerned, products in which the degree of whiteness attains or exceeds 75% with respect to the magnesium scale are preferable. Thus, the invention allows one to prepare fine powders which are particularly white, and the degrees of whiteness of which attain and even frequently exceed 90% with respect to the magnesium scale. These fine powders consist principally of hydrates and can be employed in all industries utilizing such materials as charges or fillers, for example by industries manufacturing paints, papers, plastic materials, rubber, insecticides, soaps, and washing products.
For the present invention, mixed hydates including principally at least one of the following: calcium aluminates, calcium silicates, or mixtures of the same can be prepared by co-precipitation. For example, it is convenient to prepare rapidly the following hydrates according to the invention, namely CaO; Al.sub.2 O.sub.3 ; n H.sub.2 O (8 &lt; n &lt; 13), 4 CaO, Al.sub.2 O.sub.3, n H.sub.2 O (with n usually comprised between 10 and 19); 3 CaO. Al.sub.2 O.sub.3. 6 H.sub.2 O; 2 CaO. Al.sub.2 O.sub.3. 8 H.sub.2 O; x CaO. y SiO.sub.2. z H.sub.2 O, such as 3 CaO. 2 SiO.sub.2 3 H.sub.2 O or CaO. SiO.sub.2. n H.sub.2 O referred-to as "tobermorites" (with 0.4 &lt; x/y &lt; 3 and 0.5 &lt; z/y &lt; 6 and 0.1 &lt; n &lt; 6).
All these products can be obtained alone or in mixtures, depending on the purities and proportions of the employed Portland or aluminous cements. The chemical reactions for their obtainment are known and described for example in the cited book of F. M. Lea or in "High Alumina Cements" written by T. D. Robson.